Dive theory: displacement explained

If you’re working through your PADI Divemaster or Instructor Development Course, you’ll almost certainly come across displacement questions. For many candidates, they can be confusing at first, but once you understand what’s actually happening, these questions become easy. Let’s break it down in simple terms.

What is displacement?

When you place an object in water, it pushes water out of the way. This is called displacement.

The amount of water displaced tells us how much upward buoyant force the water exerts on the object. This is based on Archimedes’ Principle: "The upward buoyant force on an object is equal to the weight of the fluid it displaces."

According to legend, King Hiero II of Syracuse asked Archimedes to determine whether his new crown was pure gold or secretly mixed with silver—without damaging it. Archimedes was stumped until he stepped into a public bath one day and noticed the water rise around him. In that moment, he realized that the volume of water displaced equals the volume of the object submerged, meaning he could compare the crown's displacement to that of pure gold to detect any fraud. Overjoyed by his discovery, he supposedly leapt from the tub and ran home naked through the streets of Syracuse shouting "Eureka!" ("I have found it!")—a word that has been the go-to exclamation for sudden insight ever since.

In diving terms:

• Heavier objects sink because they weigh more than the water they displace.

• Lighter objects float because they displace more weight in water than they themselves weigh.

The core formula

Here’s the golden rule you’ll use for every question: Buoyant Force (kg) = Weight of Water Displaced (kg)

And since 1 litre of water =

• 1 kg for fresh water

• 1.03 kg for sea water

We can easily convert litres into kilograms.

Example 1: Basic displacement

A 600 kg cement block lies in 10 metres of fresh water. The block displaces 300 litres of water. What is the minimum amount of water that must be displaced to move the block off the bottom?

Step 1 – Work out buoyant force already acting: 300 L of water displaced = 300 kg of buoyant force.

Step 2 – Work out effective weight underwater: 600 kg (block) – 300 kg (buoyant force) = 300 kg still holding it down.

Step 3 – Work out how much more buoyant force is needed: We need another 300 kg of lift to make it neutrally buoyant.

Step 4 – Convert to litres: In fresh water, 300 kg = 300 litres.

Answer: 300 litres of additional displacement needed.

Example 2: Liftbag problem

An object weighs 250 kg and displaces 40 litres of seawater. How much seawater must be displaced with a liftbag to make the object 50 kg positively buoyant?

Step 1 – Buoyant force from current displacement: 40 L seawater × 1.03 = 41.2 kg of lift.

Step 2 – Net downward force: 250 – 41.2 = 208.8 kg still holding it down.

Step 3 – To make it 50 kg positive: We want it to have 50 kg more lift than its weight, so we add:208.8 + 50 = 258.8 kg total lift needed.

Step 4 – Convert to litres of seawater: 258.8 ÷ 1.03 = ≈251.3 litres.

Answer: 251 litres of seawater must be displaced with the liftbag.

How to approach every question (the cheat sheet)

Step	What to Do	Example
1	Write down object’s weight	e.g. 250 kg
2	Calculate buoyant force from current displacement	litres × density of water
3	Subtract to find net downward force	weight – buoyant force
4	Add or remove lift as needed	depends on “neutral” or “positive” buoyancy
5	Convert result to litres	kg ÷ 1 (fresh) or 1.03 (salt)

Useful conversions:

Unit	Conversion
1 litre fresh water	1.00 kg
1 litre sea water	1.03 kg
1 m³ water	1000 litres
10 metres depth	2 bar absolute pressure

Quick tips

• Always label whether you’re in fresh or salt water.

• Keep your working steps clear

• Think in weights, not volumes – litres can always be converted later.

• Practice with different examples — it soon clicks!

Understanding displacement isn’t just about passing an exam. It’s about truly grasping how buoyancy works — a fundamental concept that makes you a better diver and a better instructor. Once you can confidently explain and calculate displacement, you’ll find buoyancy control, lift bag use, and student explanations much easier.